I would like to set up a free coupon so that I can allow a client download all the photos from an album of a particular size.
Is there a method to do that?
You are able to setup a coupon for a digital download per the SmugMug Help site:
Unfortunately I don't see a way to achieve that for an entire gallery while using coupons, short of handing out multiple coupons.
Another option that is outside of the coupon gallery is to simply setup a full Album Download and just price it as free. Just open up the custom pricing and click on the Downloads tab, then "Album Downloads".
I would suggest simply creating a new gallery with access given to only the size you wish, and providing those images downloadable for free. Skip the entire coupon piece as I don't think it handles this functionality currently.